Hybrid vehicle fuel systems may include a sealed fuel tank configured to withstand high fuel tank pressure and vacuum levels. The vehicle may include a fuel tank isolation valve to seal the fuel tank from the atmosphere. Pressure in the fuel tank may build up due to the generation of fuel vapors. If the pressure inside the fuel tank reaches the capacity of the fuel tank, fuel vapors may be released from the fuel tank into a fuel vapor canister by opening the fuel tank isolation valve. Hydrocarbons (HCs) in the fuel vapors may be adsorbed and stored in the fuel vapor canister, and the rest of the fuel vapors may be vented to atmosphere. At a later time, such as when the engine is in operation, stored HCs in the fuel vapor canister may be purged into an engine intake manifold and combusted as fuel. However, due to non-uniform purge flow within the canister, the fuel vapor canister may not be completely purged. Consequently, retained HCs may breakthrough from the fuel vapor canister and vent to the atmosphere as a bleed emission. A hybrid vehicle may in particular suffer from bleed emissions due to limited engine runtime. Further, bleed emission may be significant for a vehicle that has been parked in high ambient temperature for a long duration.
Other attempts to address bleed emissions including arranging a fuel vapor sensor at the fresh air port of the fuel vapor canister. One example approach is shown by Oemcke et al. in U.S. Pat. No. 6,293,261. Therein, fuel vapor content exiting the fuel vapor canister is monitored in real time by the fuel vapor sensor. However, the inventors herein have recognized potential issues with such systems. As one example, the fuel vapor sensor needs to be rationalized (e.g., diagnosed) in the presence of HCs. However, since the fuel vapor sensor is positioned at the fresh air port of the fuel vapor canister, the fuel vapor sensor may only detect HCs when there is HC breakthrough from the canister to the atmosphere. When the fuel vapor canister functions effectively and is thoroughly purged, fuel vapors flowing through the fuel vapor sensor may contain little or no HCs. Due to the sensor's limited exposure to HCs, degradation of the fuel vapor sensor may be left undetected. Consequently, bleed emissions at a later time may not be effectively monitored and controlled. Other approaches to rationalize the fuel vapor sensor include determining fuel vapor sensor degradation during fuel vapor canister purging. During fuel vapor canister purging, fresh air is first routed from a purge port to the vent port of the canister, and then flows desorbed HCs to a manifold of the engine via a HC sensor, such that the HC sensor rationality check may be performed as desorbed HCs flow through the HC sensor. However, in hybrid vehicles, such as a plug-in hybrid electric vehicle, engine running conditions may be infrequent. Since fuel vapor canister purging occurs during engine running, canister purge events may be rare. As such, opportunities for performing HC sensor rationality checks may be limited.
In one example, the issues described above may be addressed by a method for an engine, comprising: during loading of a fuel vapor canister, actuating one or more valves to flow fuel tank vapors from a fuel tank to a first valve coupled to a fresh air side of the fuel vapor canister; sensing hydrocarbons with the a sensor positioned in a flow path of the fuel tank vapors and fluidly coupled to fresh air; and diagnosing operation of the sensor based on the sensed hydrocarbons. In this way, degradation of the sensor may be regularly checked for proper operation despite infrequent engine on conditions.
As one example, a method for an engine comprises, loading a fuel vapor canister by flowing fuel vapors from a fuel tank to a purge port of the canister, and monitoring HC content in fuel vapors flowing from the fuel tank to the purge port of the canister by a HC sensor. As fuel vapors flow through the HC sensor, the HC sensor rationality check is performed. After finishing the rationality check, the method may include flowing fuel vapors from the fuel tank to a load port to continue loading the fuel vapor canister. Alternatively, if the HC rationality check is not required during canister loading, fuel vapor canister loading may be performed by flowing fuel vapors from the fuel tank to the load port of the canister. As such, fuel vapors may flow to the canister in a first direction or second direction during canister loading depending on whether HC sensor rationality check is requested. By performing the HC rationality check during canister loading, the HC sensor may be exposed to high levels of HCs to diagnose the sensor during engine off conditions. Thus, proper functioning of the HC sensor may be checked regularly regardless of how frequently an engine is running. As a result, detection of canister breakthrough may be improved, thereby reducing HC emissions from the fuel system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.